Semiconductor chip manufacturers have long recognized the deleterious effects of deposits, such as, for example, oxide deposits on the reaction chamber walls in which the various chemical reactions and deposition processes take place during chip manufacture. As impurities build up on reaction chamber surfaces, such as interior chamber walls, the risk increases that such impurities may be co-deposited on target work piece surfaces, such as computer chips. Therefore, such chambers must be periodically cleaned during down cycles in the chip manufacturing process.
One known way to clean the unwanted deposits from interior reaction chamber walls is to produce a fluorine plasma in the reaction chamber, under sub-atmospheric pressure, to remove unwanted silicon-containing oxide deposits from the interior chamber walls. While diatomic fluorine (F2) is an excellent candidate as a source for the fluorine plasma, it is highly reactive. Therefore, the fluorine plasma can be more safely obtained by dissociating other fluorine-containing compounds such as, for example, NF3, CF4, C2F6, SF6, etc. In essence, any fluorine-containing gas that can be decomposed into active fluorine species potentially can be used for chamber cleaning.
Nitrogen trifluoride (NF3) has proven to be an extremely safe, useful and versatile source of elemental fluorine for reactions, and for use in apparatus cleaning protocols. However, the dramatic surge in demand for NF3 has resulted in a virtual global shortage of this relatively expensive material. In addition, most of the cleaning processes using NF3 only consume about 15% of the fluorine contained within the NF3 in the actual cleaning operation, with the remaining fluorine being exhausted, treated, neutralized and eventually discarded.
Cyclical adsorption processes are generally employed for use in fluorine recycling processes. Such preferred processes include pressure swing adsorption (PSA) and temperature swing adsorption (TSA) cycles, or combinations thereof. The adsorption can be carried out in an arrangement of two or more adsorption beds arranged in parallel and operated out of phase, so that at least one bed is undergoing adsorption while another bed is being regenerated. Specific fluorine recycle applications into which the invention can be incorporated include vacuum vapor deposition and etching chamber cleaning processes, etc.
The fluorine-containing source compound, any other reagents, and inert gases used in the chamber cleaning process are typically supplied as compressed gases and are admitted into the chamber using a combination of pressure controllers and mass flow controllers to effect the cleaning process. The cleaning process itself requires that a plasma be maintained upstream of, or in the chamber to break up the fluorine-containing source compound so that active fluorine ions and radicals are present to perform the cleaning chemistry. To maintain the plasma, the chamber is kept at a low pressure, typically between about 1 and 10 Torr absolute, by using a vacuum pump to remove the gaseous waste products and any unreacted feed gases that comprise the exhaust gas. The pressure in the chamber is typically controlled by regulating the flow of exhaust gas from the chamber to the chamber pump using a vacuum throttle valve and feedback controller to maintain the chamber pressure at he desired setpoint. The chamber cleaning operation is performed intermittently between deposition operations. Typically, one to five deposition operations will be performed between every chamber cleaning operation.
In typical reaction chamber cleaning apparatuses the reaction gases are NF3 and argon. Typically the NF3 is dissociated into nitrogen and energetic fluorine radicals. However, the unused radicals recombine to form fluorine, which is directed from the system as waste and exhausted, such as to a facility abatement device.
It would be advantageous to reclaim a portion of the fluorine waste by purifying the fluorine, discarding the impurities in the waste stream and then return the fluorine for use in the cycle. However, traditional packed column separation techniques have proven unsuitable or unreliable for use with fluorine. Fluorine's high degree of reactivity and instability makes a successful adsorbent selection (for use as adsorbent packing within a separation column) extremely difficult. The adsorbent will combust, or otherwise adversely react prematurely and unpredictably unless the materials used to make the column and the adsorbent bed are made to be non-reactive with fluorine, or are protected by a stable fluoride layer. Traditional steel columns are often too reactive, and plastic vessels and beds combust easily. Silica gels and molecular sieves are also unsuitable for fluorine separation due to their lack of stability when exposed to fluorine.